I remember my first kickboxing tournament I entered. I came out and threw a light legkick and got blasted with a right hand and it had me wobbled for a second. I remember thinking to myself, what the hell, this is nothing like training!
Just because you may look like a stud in the gym or hitting pads doesn't mean you can fight. As the old saying goes, pads don't hit back.
Work as many lifelike drills as possible. sparring is a must, and learning to eat shots. If you know anything about Ali, it was said that he would let guys beat him up in sparring. In fact if you would watch him spar he didn't look like a very good boxer. He would let people hit him hard over and over. this transitioned into him defeating george foreman after taking a helacious beating for 5 rounds.
another very good drill is standing against a wall (back to the wall) and letting a partner throw strikes at you. you are not allowed to move off the wall or move left and right. all you can do is defend to the best of your ability. Work your shell, head movement, parry/catch etc. This will greatly help your defence in the long run.
I had a muay thai coach years ago who taught me most of what I know. Once he started seriously training me he gave me a very good tip.. He said that when you walk around in daily life and hurt yourself (stub your toe, etc) don't scream out in pain and swear. Just breath and ignore it. Pain is nothing, learn to toughen yourself up in every aspect, mental strength > physical strength.
KICKBOXING AND MUAY THAI
When you are training for your first fight the most important strike/strikes I would say to learn are your jab and your push kick.
when you have your first few fights no matter how well you trained or how many times you have competed before, they will be a blur. Usually the first time someone gets hit hard their gameplan goes out the window. Learning a wicked teep or pushkick will help you keep the fight at your pace as well as stop the shoot if your opponent is far enough back.
I have seen fights where guys are throwing spinning backfists, hook kicks etc and they have shitty roundhouse kicks and even worse push kicks... Learn the fundamentals of every martial art before you tackle the flashy stuff.
Another tip I believe to be very important is too work throwing a roundhouse kick everytime you break a clinch. Everytime I clinch in training I make sure I am the one to disengage and I make sure I throw a head kick on my way out. If the guys hands are down you are going to catch him.
WRESTLING
If i could change any one thing in my martial arts experience, I would have learned to wrestle before anything else. being able to dictate where the fight takes place is so important. Wrestlers who learn to strike and learn to stop subs are nearly unstoppable.
Learn to get comfortable with the distance of a shot and learn to time strikes behind it. A good jab into a double leg always works well.
Another good drill is to do muay thai rounds while a third person kicks an excercise ball at you. every time the ball gets close to you, sprawl on top of it. Bounce back up and kick the ball back and continue on with your rounds. This will help you learn the speed and explosiveness you need to have a good sprawl.
LEARN WRESTLING
BJJ
This can be a tricky one to master, it is such an art that people get sloppy and can get subbed while trying for a sub of their own.
learn the basics, rear naked choke, triangle, armbar from guard, from mount etc. Kimura from side control as well as an americana.
Learn how to defend them 10x as good as you learn how to execute them. (my subs are awful but my sub defence is far better)
Guillotines can work well but the problem is that when an un experienced guy goes for one he tends to drop to guard as soon as he gets it. the opponent escapes and you end up on your back.
If you get a guillotine DO NOT DROP TO YOUR BACK. try to finish it standing, this way if you fail with it at least you can go to a thai clinch and blast some knees.
POSITION BEFORE SUBMISSION
BJ Penn is an amazing example of this. People don't realize how good his jujitsu actually is, He doesn't use flashy subs ever. but if he gets your back your fucked. I remember talking to Joe daddy in montreal after one of the UFC's and he told me he had never felt control like that before when BJ got ahold of him.